The envelope glycoprotein (Env) of HIV is a bipartite protein consisting of a surface subunit (gp120) that has receptor binding activity and a non-covalently associated transmembrane subunit (gp41) that has membrane fusion activity. Gp120 binding to cellular receptors triggers conformational changes that activate the fusion activity of gp41, which mediates delivery of the viral genome across the cell membrane. Our research goals are to understand structure-function relationships in the envelope glycoprotein in order to provide information for the rational design of novel HIV inhibitors and vaccines. We have undertaken mutagenesis studies (Weng and Weiss 1998 J. Virol. 72:9676-9682) of a highly conserved coiled domain gp41 that has been proposed to be a good target for the development of HIV inhibitors and vaccines. We found that most non-helix disrupting mutations abolish gp41 fusion activity indicating that drugs or antibodies directed to this region are unlikely to generate resistant mutants. However, two residues in positions that line a hydrophobic pocket of the coiled coil tolerate mutations suggesting that this region may be more susceptible to escape from inhibition. We are currently undertaking further mutagenesis to define the requirements for residues packing in the cavity and how that affects the fusion activity of gp41. This information contributes to a foundation for the rational design of novel inhibitors and vaccines targeting the gp41 coiled-coil region.